I think I've ever read in my virology book that influenza virus is truly can change its two protein markers which are hemaglutinin (H) and neuraminidase (N). And this changing process is done in two ways which are via genetic drift and via genetic shift whereas in genetic shift, the virus really develops a fully brandnew protein marker whether H or N, so there're many possibilities for the influenza A's subtipes.

Q: Why are chemists great for solving problems?
A: They have all the solutions.